Cell, Vol. 46, 25-37, January 16, 1967, Copyright 0 1967 by Cell Press sdc-I: A Link between Sex Determination and Dosage Compensation in C. elegans Anne M. Villeneuve and Barbara J. Meyer Department of Biology Massachusetts Institute of Technology Cambridge, Massachusetts 02139 Mutations in the X-linked gene sdc-T affect both sex determination and X-chromosome dosage compensa- tion in C. elegans, providing evidence that these two pathways share a common step. In XX animals (nor- mally hermaphrodites), sdc-7 mutations cause partial masculinization and elevated levels of X-linked gene expression, an apparent shift of both pathways toward their X0 modes of expression. The masculinization occurs through effects on the major sex determlna- tion pathway, upstream of all prwiously identified sex- determining genes. X0 animals are apparently unaf- fected by the sdc-7 mutations. We propose a model in which the wild-type sdc-7 activity is either a compo- nent of the primary sex-determining signal (the X/Au- tosome ratio) or involved in transmitting information about this signal to both the sex determination and dosage compensation pathways. Introduction The primary sex-determining signal in Caenorhabditis elegans is the ratio of X chromosomes to sets of auto- somes (X/A) (Nigon, 1951; Madl and Herman, 1979). Diploid nematodes with two X chromosomes (XX, X/A = 1.O) develop as self-fertilizing hermaphrodites, while those with a single X chromosome (X0, X/A = 0.5) develop as males. Since the sole genetic difference between the two sexes is the number of X chromosomes, C. elegans must possess a mechanism to assess the relative number of X chromosomes in order to direct the organism along one of two alternative developmental pathways. Although the mechanism of X/A ratio assessment is not yet known, several autosomal regulatory genes respon- sive to this primary signal have been identified as essen- tial for either hermaphrodite development (tra genes) or male development (her and fern genes). Mutations in these genes result in discrete transformations in sexual fate, causing, for example, XX animals to develop as fertile males or X0 animals to develop as fertile hermaphrodites (Klass et al., 1976; Hodgkin and Brenner, 1977; Nelson et al., 1978; Hodgkin, 1980; Kimble et al., 1984). The analysis of interactions between these mutations has led to the pro- posal of a genetic pathway for C. elegans sex determina- tion in which the activities of these genes are regulated sequentially to implement the choice of sexual fate desig- nated by the X/A signal (Hodgkin et al., 1985). As a consequence of the primary sex determination sig- nal, males (X0) and hermaphrodites (XX) carry different doses of X-linked genes. The nematode possesses a mechanism to compensate for this difference in X-linked gene dosage, equalizing the levels of X-specific mRNA transcripts in XX and X0 animals (Meyer and Casson, 1986). This dosage compensation process depends on X-chromosome dosage per se and is independent of sex- ual morphology; hence, compensation apparently occurs even in nematodes whose sex has been transformed by a mutation in a sex-determining gene (Meyer and Casson, 1986). Mutations in several genes have been shown to dis- rupt dosage compensation in C. elegans, resulting in elevated X-specific mRNA transcript levels in XX animals (Meyer and Casson, 1986). Whether the nematode uses a unitary mechanism based on the X/A signal to trigger both the choice of sex- ual fate and the appropriate level of X-linked gene expres- sion is an important unresolved issue. If a single mecha- nism operates in the initial steps of both sex determination and dosage compensation, genes involved in this mecha- nism should be defined by single mutations that disrupt both processes. We have found that mutations in the X-linked gene s&-l (sex determination and dosage com- pensation) have precisely these effects. Specifically, they result in both masculinization and elevated levels of X-spe- cific gene expression in XX animals. The experiments presented in this paper suggest that s&-l functions at a common step shared by the sex determination and dos- age compensation pathways. Results sdc-I Mutations Cause Variable Sexual Transformation of XX Animals Two independently isolated recessive mutant alleles of the X-linked gene s&-l result in very similar phenotypes. sdc- l(n485)-previously called egl-76-was originally identi- fied on the basis of a defect in hermaphrodite egg laying (Egl) (Trent et al., 1983). The second allele, sdc-7(y4), was isolated following EMS mutagenesis on the basis of both its Egl and sexual transformation (lra) phenotypes (J. Plenefisch and B. Meyer, unpublished data). XX her- maphrodites homozygous for either of these mutations are shorter than wild-type, contain late-stage eggs, and of- ten have abnormal protruding vulvae (Figure lc). (As we will discuss in detail below, these Egl and short pheno- types appear to result from a defect in X-chromosome dos- age compensation). Occasional (<l%) sexually trans- formed XX animals with male-like tail structures, including partial structural rays and fans, are present in cultures of these strains. These sdc-7 mutant phenotypes exhibit par- tial maternal rescue, such that the majority of homozy- gous sdc-7 XX progeny of heterozygous (sdc-7/+) mothers appear wild-type, while homozygous sdc-7 progeny of homozygous mothers are mutant. sdc-l(n485) and sdc- l(y4) X0 animals are phenotypically wild-type males that mate efficiently. XX animals that carry sdc-l(n485 or y4) in Pans to a chromosomal deficiency of the locus (sdc-7hnDf7) are